1. Field of the Invention
The present invention relates to managing access to data stored on tape media and more particularly, reordering User Data Segments (UDSs) in a list to reduce UDS seek times.
2. Description of the Related Art
The capacity of tape media used for data storage continues to increase for a corresponding form factor. For example, the native capacity of Linear Tape-Open (LTO) Generation 5 media is 1.5 terabytes (TB). As capacity of tape media has increased, its usage has changed. In the past, tape media has been primarily used for data back-up and archiving. Currently, it is also being used for a Virtual Tape Server (VTS), Hierarchical Storage Management (HSM), and file systems such as Long Term File System (LTFS).
The data transfer rate of tape media has also continued to increase. As an example the native read and write transfer rate of the previously-referenced LTO Generation 5 tape drives is 140 MB/second. In part, increases in data transfer rates are achieved through the use of data compression approaches. However, data seek times for an individual record on an LTO Generation 5 tape drive averages 1 minute. One reason for the long seek time is the combination of the length of LTO Generation 5 tape media, which is 800 meters, and its corresponding reposition velocity of approximately 10 meters/second. Another reason is that a tape drive typically does not know, with certainty, the actual physical position of a target record on the tape medium.
While long seek times are generally considered one of the weak points of a tape drive, they do not present too much of a disadvantage when accessing an individual record, or a sequence of records whose physical position are serially located on a tape media. However, long seek times can present additional latency when reading a sequence of target records that are not serially located on a tape media. In these situations, the tape drive is required to wind, and unwind, the tape media to search for a target record's location. As a result, the total amount of time it takes to read a sequence of non-serialized records can increase dramatically as the number of target records grows.